High voltage electrolytic capacitors are often employed in implantable medical devices. These capacitors are required to have a high energy density because it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (“ICD”), also referred to as an implantable defibrillator, because the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume. ICDs typically use two to four electrolytic capacitors in series to achieve the desired high voltage for shock delivery. Typically, metal foils (e.g., aluminum foil) are used in the electrolytic capacitor due to their small size. Because the electrostatic capacitance of the capacitor is proportional to its electrode area, the surface of the metallic foil may be, prior to the formation of the dielectric film, roughened or subjected to a chemical conversion to increase its effective area. This step of roughening the surface of the metallic foil is called etching. Etching is normally carried out either by the method (chemical etching) of conducting immersion into a solution of hydrochloric acid or by the method (electrochemical etching) of carrying out electrolysis in an aqueous solution of hydrochloric acid. The capacitance of the electrolytic capacitor is determined by the extent of roughing (the surface area) of the anode foil and the thickness and the dielectric constant of the oxide film.
Due to the limited surface area that may be provided by etching metallic foils, attempts have also been made to employ porous sintered pellets in wet electrolytic capacitors—i.e., “wet tantalum” capacitors. The anode of a typical wet electrolytic capacitor includes a porous anode body, with an anode leadwire extending beyond the anode body. The anode can be formed by first pressing a tantalum powder into a pellet that is then sintered to create fused connections between individual powder particles. Unfortunately, when forming such anodes for high voltage medical applications, problems are often experienced. Namely, in such anodes, it is often difficult to create a sufficient number of contacts between the anode leadwire and the tantalum powder particles, which can result in an increase in equivalent series resistance (ESR).
As such, a need currently exists for an improved anode for use in high voltage capacitor, such as those employed in implantable medical devices.